Spontaneous opal glasses, viz., glasses which opacify upon being cooled from the melt, have been known to the glass art for many years, especially in such consumer applications as tableware. U.S. Pat. No. 3,661,601 provides a concise review of the various mechanisms involved in the development of spontaneous opal glasses and reference is made to that patent for those details.
As is explained in that patent, one type of spontaneous opal is characterized by the separation of a second glassy phase, i.e., not a crystalline phase, which demonstrates a refractive index different from that of the parent or original glass. In essence, this type of opal glass derives its opacity from immiscible droplets dispersed within the parent glass. The patent discloses one group of such glasses consisting essentially, as expressed in weight percent on the oxide basis, of
______________________________________ SiO.sub.2 50-75 Al.sub.2 O.sub.3 3-9 CaO 11-20 B.sub.2 O.sub.3 1-7 Na.sub.2 O 0-7 K.sub.2 O 0-7 Na.sub.2 O + K.sub.2 O 3-10 F 2-4 ______________________________________
U.S. Pat. No. 3,673,049 describes the production of laminated glass articles wherein each lamina displays a state of stress which is opposite to the state of stress exhibited by the adjacent layer(s). Phrased in a different manner, the patented articles are composed of laminae demonstrating tensile and compressive stress in alternating relationship with each other. Where a three-ply laminated product is formed comprising a relatively thick interior or core portion enclosed within a thin surface or skin layer manifesting a lower coefficient of thermal expansion than the interior portion, the surface plies will be under compression and the interior portion under tension. This surface compression layer confers significantly enhanced overall mechanical strength to the resultant article.
Three-ply laminates of thin cross section, but displaying high mechanical strength, have been marketed as tableware by Corning Glass Works, Corning, N.Y., under the trademark CORELLE.RTM.. Such products consist of a relatively thick core portion composed of a spontaneous opal glass and a transparent skin glass, the skin glass being relatively thin in cross section and possessing a lower coefficient of thermal expansion than the opal interior portion.
U.S. Pat. No. 3,673,049 reports a group of spontaneous opal glass compositions asserted to be particularly useful as interior portions for three-layer laminates. Those glasses are stated to consist essentially, expressed in weight percent on the oxide basis, of
______________________________________ SiO.sub.2 50-75 Al.sub.2 O.sub.3 3-20 Alkali Metal Oxide 3-20 Alkaline Earth Metal Oxide 0-20 B.sub.2 O.sub.3 + CeO.sub.2 + Bi.sub.2 O.sub.3 + PbO + GeO.sub.2 + CdO 0-10 ZnO + Ta.sub.2 O.sub.5 + ZrO.sub.2 + TiO.sub.2 + La.sub.2 O.sub.3 As.sub.2 O.sub.3 + Sb.sub.2 O.sub.3 0-2 Cl 0-1.5 NiO + V.sub.2 O.sub.5 + Nd.sub.2 O.sub.3 + CuO + CoO + Fe.sub.2 O.sub.3 0-5 MnO.sub.2 + Cr.sub.2 O.sub.3 F 3.8 ______________________________________
U.S. Pat. No. 3,737,294 is also drawn to laminated glass articles and discloses the same group of spontaneous opal glass compositions as that reported by U.S. Pat. No. 3,673,049 as being especially suitable for use as interior portions for three-ply laminated products.
U.S. Pat. No. 3,649,440 is directed to the thermal tempering of multi-ply laminated glass bodies, wherein adjacent layers are alternately in compression and tension, to substantially increase the impact resistance of the bodies and inhibit spontaneous breakage witnessed as a result of bruise checks arising from mechanical abuse during use. The patent discloses three-layer laminates utilizing a spontaneous opal glass as the interior portion. No glass compositions per se are claimed but operable ranges calculated from the specific exemplary opal glasses tabulated in the patent are set out below, expressed in weight percent on the oxide basis:
______________________________________ SiO.sub.2 64-66.4 Al.sub.2 O.sub.3 6.2-6.3 B.sub.2 O.sub.3 1.3-4.5 MgO 0.7-0.9 CaO 13.6-15.7 Na.sub.2 O 2.1-4.4 K.sub.2 O 1.5-4.1 F 2.7-4.4 ______________________________________
The use of colorant ions, such as the transition metals (Co, Cr, Cu, Fe, Mn, Ni, Sn, Ti, and V) and the rare earths (Ce, Dy, Er, Eu, Ho, Nd, Pr, Sm, and U), either individually or in various combinations, is well-recognized in the glass art to impart tints to opal and transparent glasses. The color obtained from any individual colorant is influenced, inter alia, by the chemistry and composition of the glass, by the presence of non-oxide anions, e.g., Cl and S, by the thermal history the glass experiences, e.g., thermal tempering, and by the occurrence of an amorphous phase separation or crystallization taking place in the glass.
Furthermore, a spontaneous opal glass commercially practical for use as a core glass in a three-layer glass laminate suitable for tableware must satisfy a matrix of melting, forming, physical, and chemical properties. For example, the glass is required to display the necessary viscosity and liquidus parameters to allow formation of the laminated article via bringing a stream thereof into contact with a stream of the skin glass. The glass must resist devitrification after relatively long term contact with platinum and refractory forming members and melting unit elements. The opacity of the glass should be densely opaque in thin cross section and remain so even if the glass is subjected to subsequent heat treatments such as may be encountered in decorating.
U.S. application Ser. No. 254,901, filed Apr. 16, 1981 in the names of H. J. Holland and J. E. Megles, Jr., discloses spontaneous opal glass compositions suitable for use as core portions in CORELLE.RTM. brand tableware. Those glasses consist essentially, as analyzed in weight percent on the oxide basis, of about
______________________________________ SiO.sub.2 64.5 .+-. 2.0 Al.sub.2 O.sub.3 6.25 .+-. 0.5 Na.sub.2 O 3.0 .+-. 0.5 K.sub.2 O 3.0 .+-. 0.5 MgO 1.25 .+-. 0.3 B.sub.2 O.sub.3 4.5 .+-. 0.7 F 3.25 .+-. 0.5 CaO 14.5 .+-. 1.0 ______________________________________
wherein the preferred MgO content varies between about 1.05.+-.0.05%.
The use of Ni ions as a colorant is not uncommon. The tint resulting can range from yellow-through-brown to purple, depending upon the atomic structure of the host base glass, as well as the thermal history or fictive temperature of the glass. This phenomenon is examined by W. A. Weyl in Coloured Glasses, Dawson's of Pall Mall, London, 1959, and in U.S. Pat. No. 4,235,634 where the use of a combination of nickel and manganese ions to color soda lime glasses having compositions within very narrowly defined ranges is described. U.S. Pat. NOs. 3,661,601 and 3,673,049, supra, also disclose the fact that opal glasses can be colored through the addition of nickel ions.
Colored, three-ply laminates can be prepared by incorporating nickel ions in the opal core glass, in the transparent skin glass, or both. It has been found especially advantageous to include a colorant in the core glass since greater homogeneity of appearance is achieved. Moreover, to secure a similar color density by tinting the skin only, much higher levels of a colorant are demanded. As a result, small differences in skin or surface layer thickness can give rise to noticeable inhomogeneities, streaks, etc. in the laminated ware. Consequently, tinting the core only imparts the most homogeneous appearance to the products.
Marketing studies have indicated that only about one-third of the tableware purchased in the United States utilizes products having a white background. The presently available CORELLE.RTM. brand ware involves a white body with a number of decorations applied thereto. Therefore, the primary objective of the instant invention was to provide a spontaneous opal core glass for a three-ply laminate having melting, forming, chemical, and physical properties similar to those exhibited by the core glass of CORELLE.RTM. brand ware, but which would have a tint therein that could range from ivory-through-beige-through-tan-to-brown, depending upon the color sought. Stated more specifically, the most desired goal would be to incorporate a colorant in the composition, expressed in weight percent on the oxide basis, of about
______________________________________ SiO.sub.2 64.5 .+-. 2.0 Al.sub.2 O.sub.3 6.25 .+-. 0.5 Na.sub.2 O 3.0 .+-. 0.5 K.sub.2 O 3.0 .+-. 0.5 MgO 1.25 .+-. 0.3 B.sub.2 O.sub.3 4.5 .+-. 0.7 F 3.25 .+-. 0.5 CaO 14.5 .+-. 1.0 ______________________________________
in an amount sufficient to confer that tint, but not in such quantity as to significantly alter the fundamental characteristics of the base glass.